The present invention relates to a static pressure non-contact type mechanical seal suitable for use in such rotary machines as turbines, blowers, compressors, agitators and rotary valves handling a variety of gases, including poisonous gases, inflammable gases, explosive gases and powder-mixed gases.
A known non-contacting mechanical seal of the non-contact type comprises a stationary seal ring fixed on a seal case, a spring retainer provided on a rotary shaft, a rotary seal ring mounted on the rotary shaft to be movable in the axial direction and held between the stationary seal ring and the spring retainer with O rings provided as a secondary seal for sealing a space between the rotary shaft and the rotary seal ring, seal gas supply passages that pass through the stationary seal ring and supply a gasxe2x80x94under a higher pressure than the fluid to be sealedxe2x80x94between the seal end faces facing each other of the two seal rings, and spring members, placed between the rotary seal ring and the spring retainer, pressing the rotary seal ring against the stationary seal ring, wherein the two seal rings are relatively rotated with the opposed seal end faces held in a non-contacting state by the seal gas supplied therebetween, thereby producing a seal between the inner and outer circumferential regions of the relatively rotating parts, that is, the sealed fluid region and the outside region (usually, the atmospheric region).
In the non-contact-type mechanical seal of such a construction, a static pressure fluid film of the seal gas is formed between the two seal end faces, and the presence of this fluid film holds the two seal rings in a non-contacting state, that is, keeps the two seal rings only slightly spaced from each other. The seal gas supplied between the two seal end faces is under a higher pressure than the fluid to be sealed. Therefore, the seal gas leaks out to the sealed fluid region and the outside region, but the sealed fluid cannot get between the two seal end faces. Thus, the sealed fluid is perfectly prevented from leaking out to the outside region. And the shaft can be sealed well in the rotary machine handling such fluids as poisonous gases, inflammable gases and explosive gases that must not leak out.
In such static pressure non-contact-type mechanical seals, self-excited vibration called pneumatic hammer vibration is inevitably caused at the seal gas flowing passages up to between the two seal end faces, because the seal gas supplied from the seal gas supply passages to the seal end faces is a compressed gas. While the stationary seal ring clamped on the seal case is not affected by that, the rotary seal ring that is held on the rotary shaft merely via the O rings vibrates with a minute amplitude as small as or smaller than the gap between the seal end faces. The vibration of the rotary seal ring has no adverse effect in particular on the seal function of the non-contact-type mechanical seal, but it is desirable that the vibration should be prevented so as not to cause a vibration sound.
It is an object of the present invention to provide a static pressure non-contact mechanical seal that can prevent the rotary seal ring from vibrating to the maximum possible extent.
The non-contact mechanical seal according to the present invention comprises a stationary seal ring (3) clamped on a seal case (2), a spring retainer (5) provided on a rotary shaft (4), a rotary seal ring (6) fit over the rotary shaft, movable in the axial direction and held between the stationary seal ring (3) and the spring retainer (5) with a secondary seal provided between the rotary shaft (4) and the rotary seal ring (6), a seal gas supply mechanism (9) that supplies the seal gasxe2x80x94under a higher pressure than the sealed fluidxe2x80x94to between opposed seal end faces (3a, 6a)of the two seal rings (3, 6) through one or more seal gas supply passages (15, 16, 18) that pass through the stationary seal ring (3), and spring members (7), placed between the rotary seal ring (6) and the spring retainer (5), pressing the rotary seal ring (6) against stationary seal ring (3). The two seal end faces (3a, 6a) are rotated relative to one another and held in a non-contacting state by seal gas (8) supplied therebetween, thereby producing a seal between the inner and outer circumferential regions of the relatively rotating parts, that is, a sealed fluid region (G) and an outside region (A). To achieve the aforesaid object, a cylindrical holder portion (13) surrounding the outer circumferential portion of the rotary seal ring (6) is formed on the spring retainer (5), and a pair of O rings (22) are placed and engaged in annular O ring grooves (21) slightly spaced from each other in the axial direction which are formed on an outer a circumferential portion of the rotary seal ring (6). In this manner, an annular space (23) is formed between the rotary seal ring (6) and a holder portion (13) of the spring retainer (5). And one or more seal gas leading passages (24) through which spaces between the seal end faces (3a, 6a)communicate with the aforesaid annular space (23) are formed in the rotary seal ring (6).
In a preferred embodiment, the seal gas is properly selected according to the sealing conditions. That is, the seal gas to be used should be one that is harmless even if the gas leaks out to the sealed fluid region (G) and the outside region (A) and has no adverse effect on the gas in the machine, that is, the sealed fluid. Generally, clean nitrogen gas is used as seal gas (8) that is inert to a variety of substances and harmless to humans. The O rings (22) used to seal the annular space (23) are preferably made of a non-compressive elastic material like natural rubber and synthetic rubber.
The seal gas leading passages (24) pass through the rotary seal ring (6), each passage having one open end (24a) opening on the seal end face (6a)and another open end (24b) opening into the annular space (23). The one open end (24a) of each seal gas leading passage (24) is preferably positioned exactly opposite to the static pressure generating groove (15). It is further desirable that the diameter (D) of the one open end (24a) be set to be equal to or slightly smaller than the groove width (W). To be exact, the diameter (D) means the width of the one open end (24a) in the radial direction of the rotary seal ring (6), for the one open end (24a) is not always circular in form.
The seal gas supply passages comprise static pressure generating grooves (15) formed on the seal end face (3a) of the stationary seal ring (3), a communicating space (16) formed between the stationary seal ring (3) and the seal case (2), a passage (17) on the seal case side that passes through the seal case (2) to the communicating space (16), and a passage (18) on the seal ring side that passes through the stationary seal ring (3) and extends from the communicating space (16) to the static pressure generating grooves (15). A squeezer (19) is provided in this seal gas supply passage (18). The squeezer (19) is an orifice, capillary tube or a porous material.
In a preferred embodiment, the static pressure generating grooves (15) are formed of a plurality of arc-shaped recessed grooves (15a) arranged in an annular form concentrically with the stationary seal end face (3a). The respective arc-shaped recessed grooves (15a) are identical in groove width and depth. The length (L) in the circumferential direction of the stationary seal end face portion (15b) between the arc-shaped recessed grooves (15a) is set to be equal to or about equal to the groove width (W) of the static pressure generating grooves (15) or the groove width (W) of the arc-shaped recessed groove (15a).
For reasons described below, it is desirable that the pressure (Ps) of the seal gas (8) supplied to the seal gas supply passages from a seal gas source is so controlled that the pressure (Pp)xe2x80x94the pressure in the static pressure generating groovexe2x80x94of the seal gas (8) led to static pressure generating grooves (15) through squeezer (19) is 0.5-1.5 bar higher than the pressure (Pg) of the sealed fluid. In other words, it is desirable that the supply pressure (Ps) of the seal gas (8) should be maintained 1 to 3 bar higher the pressure (Pg) of the sealed fluid.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of The Preferred Embodiments, which follows, when considered together with the attached Drawings, described below.